The invention relates to a method for calibrating a digital/analog converter and to a digital/analog converter.
With respect to the general background of D/A converters, reference is generally made to U.S. Pat. Nos. 6,346,901 B1, 4,712,091 and 5,293,166. With respect to the general background of D/A converters with online self-calibration, reference is made to the publication by D. W. J. Groeneveld, H. J. Schouwenaars, H. A. H. Termeer, C. A. A. Bastiaansen, “A Self-Calibration Technique for Monolithic High-Resolution D/A Converters”, IEEE Journal of Solid-State Circuits, volume 24, December 1989.
A digital/analog converter, also called D/A converter for short in the text which follows, is designed for converting a digital input signal into an analog output signal. Although it is basically applicable to any digital/analog converter, the present invention and the problems on which it is based will be explained in the text which follows with reference to a monolithic integrated D/A converter designed for high speed applications, with a facility for online self-calibration. By online calibration is meant that the calibration can be performed during the operation of the D/A converter, that is to say virtually in the background without a current D/A conversion having to be interrupted.
A monolithic integrated D/A converter typically has a multiplicity of converter cells arranged in a converter matrix or a so-called converter array. The individual converter cells are ideally identical in their configuration. A problem inherent in almost all monolithic integrated D/A converters consists in that typically mismatches exist between the individual converter cells which become noticeable as distortions in the spectrum of the analog output signal. The mismatches become evermore predominant with increasing integration, that is to say with increasing reduction of the size of the patterns located on the integrated circuit and can only be reduced at the cost of lesser integration and thus a larger chip area. Apart from higher costs for the D/A converter, it would also lead to a lower speed of the conversion and thus to a lower performance of the D/A converter.
To implement very high-quality communication systems with digital signal processing such as are used, for example, in mobile radio and for broadband applications, D/A converters with a medium or high sampling rate and the best possible analog characteristics are used. The quality and accuracy of such high-speed D/A converters depend on a multiplicity of different factors, of which the so-called spurious free dynamic range (SFDR) of the D/A converter represents a very decisive characteristic.
FIG. 1 shows a typical output spectrum AS which exhibits distortions in the output spectrum caused by mismatches of the D/A converter cells. In FIG. 1, the wavy line A designates the quantization noise. Apart from the frequency Fin of the input signal, there are also harmonics 2Fin, 3Fin at multiples of the frequency Fin. These harmonics 2Fin, 3Fin limit the interference-free dynamic range SFDR of the D/A converter which leads to a lesser effective resolution overall. As can be seen from FIG. 1, the interference-free dynamic range SFDR designates the difference between the maximum amplitude of the frequency Fin of the input signal and the amplitude of harmonic frequency component 2Fin which has the greatest amplitude among the harmonics 2Fin, 3Fin.
FIG. 2 will now be used to describe a calibration method, known, for example, from the IEEE publication described initially, which can be used for enlarging the interference-free dynamic range SFDR. The example in FIG. 2 shows the calibration principle by means of a 6-bit D/A converter B which thus exhibits 63 converter cells C largely of the same structure. For the calibration, the D/A converter B also has a redundant converter cell D (shown shaded in FIG. 2, cell 64) and a reference cell, not shown. The reference cell is used for the self-calibration in order to successively calibrate all converter cells of the D/A converter B. By using the redundant converter cell D, the self-calibration can be performed online, that is to say also during the operation of the D/A converter B.
In the example in FIG. 2, a total of 64 calibration cycles K1-K64, of which only the first three K1-K3 and the last one K64 have been shown in FIG. 2, are provided for calibrating the converter cells C, D of the D/A converter B. Passing through all calibration cycles K1-K64 defines a so-called calibration loop E. Within the calibration loop E, all converter cells C, beginning with the first converter cell, are successively calibrated including the redundant converter cell D. The calibration method then typically jumps back to the first converter cell in order to recalibrate the converter cells in the next calibration loop E.
The calibration of a respective converter cell C, D requires a calibration period T1-T64. This calibration period T1-T64 is predetermined for each converter cell C, D within the calibration loop and is thus constant. The calibration periods T1-T64 allocated to all converter cells C, D are thus equal.
During the determination of the calibration period T1-T64, the following must be observed: on the one hand, the calibration period T1-T64 must not be too small since otherwise the value of a respective converter element C, D to be calibrated cannot be properly corrected. On the other hand, the calibration period T1-T64 must also be selected to be not too large since otherwise the storage element, in which the difference between the value of the converter cell to be calibrated and the value of the corresponding reference cell is stored, loses the stored difference value and thus the entire calibration process would become ineffective. For this reason, the calibration period T1-T64 must be selected to be within a certain range which takes into account the two above boundary conditions and is thus selected to be not too small and not too large.
In this manner, it is possible to reduce a distortion in the output spectrum, caused by a mismatch of the converter cells within the converter array. However, this procedure has the disadvantage that, as a result, additional interfering frequencies are generated (see FIG. 1). It is found that, although the amplitudes of the harmonic frequencies 2Fin, 3Fin are reduced by the calibration, additional interfering frequencies are also generated at the calibration frequency Fcal and multiples thereof 2Fcal, 3Fcal due to the calibration. These additional interfering frequencies Fcal, 2Fcal, 3Fcal prevent a further overall improvement in the interference-free dynamic range SFDR.